The present invention relates to a protective circuit for a series circuit of a power semiconductor and an inductive load, to which a freewheeling circuit of a freewheeling diode and a reverse polarity protection semiconductor switch connected in series is connected in parallel, the power semiconductor output stage being switchable on and off depending on a predetermined setpoint value with a pulse-width modulated control signal, and the reverse polarity protection semiconductor switch can be made non-conducting via a charging pump when the polarity of the supply voltage is reversed.
The protective circuits referred to above (with defect-free components) may provide sufficient reverse polarity protection, that is, protection of the components against overloading and destruction in the case of supply voltage applied with reverse polarity. In order to obtain an effective reverse polarity protection, an additional power semiconductor switch may be connected to the freewheeling circuit. This additional semiconductor switch is controlled by the loading pump so that the semiconductor switch is conducting in the freewheeling circuit in normal operation, while it interrupts the current flow via the freewheeling diode through the non-conducting state in the case of reverse polarity supply voltage.
If irregularities which do not control the semiconductor switch in this way appear in the circuit, then uncontrolled over-voltages which result in the destruction of components appear in the circuit. This defect can result in fires, which, in particular in the case of the use of the circuit in a motor vehicle, can lead to vehicle fires.
An object of an exemplary embodiment of the present invention is to improve a protective switch of the type referred to so that defects of the loading pump or of the reversed polarity protection semiconductor switch, which may lead to the freewheeling circuit not functioning correctly, can be recognized and protective measures can be taken to protect the components of the circuit against destruction.
It is believed that the object may be achieved according to an exemplary embodiment of the present invention by the fact that a point in the freewheeling circuit of the freewheeling diode and the reverse polarity protection semiconductor switch or in the series circuit of load and power semiconductor output stage is monitored for a voltage or a current which appears (fails) in the case of non-functioning of the freewheeling circuit, and that the shut-off of the control of the power semiconductor output stage can be initiated with this voltage or this current failure.
Non-operation of the freewheeling circuit (that is, a voltage increase over a predetermined voltage or current failure) is recognized with this voltage or current monitoring, and the control of the power semiconductor output stage is switched off so that the critical components in the circuit are reliably protected against overloading. The power semiconductor output stage then is always non-conducting, that is high-resistance.
According to another exemplary embodiment, the control of the power semiconductor output stage takes place by the power semiconductor output stage being controllable via a PWM control, and by the increased voltage or current failure which appears in the case of a defect switching off the PWM control in the freewheeling circuit.
In order to obtain absolute protection against destruction of components, it is believed that switching off the PWM control at a predetermined time equal to or less than 10 ms after the appearance of the defect may be advantageous.
If a buffer capacitor is connected at the connecting point between the freewheeling diode and the reverse polarity semiconductor circuit in order to largely eliminate feedback effects of the timed operation of the load on the supply voltage, the voltage across the buffer capacitor is monitored for a rise over a predetermined value. The increased voltage across the buffer capacitor is transferred as a shut-off signal to the PWM control. In this case, in order to simplify the shutoff of the PWM control, the increased voltage across the buffer capacitor can be transferred to the PWM control as a digital shut-off signal via an analog-digital converter.
The transfer of the shut-off signal from the buffer capacitor also can be performed according to another exemplary embodiment so that the voltage across the buffer capacitor is reduced via a voltage divider and can be transferred to the analog-to-digital converter smoothed again.
A specific response threshold in the monitoring circuit is achieved according to another exemplary embodiment by comparing the voltage across the buffer capacitor or the reduced voltage in a comparator with a predetermined reference voltage, and the comparator routes a digital shut-off signal to the PWM control when the voltage across the buffer capacitor or the reduced voltage exceeds the reference voltage.
The PWM control may then be xe2x80x9csafelyxe2x80x9d shut off when the refinement is made so that the comparator is provided with an automatic circuit breaker, which maintains the shut-off state and the stable shut-off state of the comparator be canceled by a reset signal which can be supplied to the comparator from the PWM control. The reset signal switches the circuit back for operation after elimination of the defect.
A voltage monitoring may also be achieved by monitoring the drain-ground voltage of the power semiconductor output stage in a low side circuit, by having the drain-ground voltage in a comparator comparable with a predetermined reference voltage, by having the comparator supply a digital shut-off signal to the PWM control when the drain-ground voltage exceeds the reference voltage, and by interrupting the control of the power semiconductor output stage with the shut-off signal and by the reference voltage for the comparator being switched to become inactive. With this protective circuit, the power semiconductor output stage is incorporated in the protection circuit.
A voltage monitoring takes place since the freewheeling circuit is monitored for failure of the timed freewheeling current.